In early 1205, through John's influence, Peter was elected to the see of Winchester.[3] His election was disputed but, on appeal, confirmed on 25 September 1205 by Pope Innocent III.[4] Peter was consecrated on 24 March 1206.[3] Nonetheless, the new bishop stood by John's side during the whole period of the papal interdict set upon him due to John's refusal to accept Stephen Langton as Archbishop of Canterbury despite Papal warning.

At the battle of Lincoln in 1217 Peter led a division of the royal army and earned some distinction by his valour; but he played a secondary part in the government so long as William Marshal held the regency. After Marshal's death in 1219 Peter led the baronial opposition to Hubert de Burgh, with varying success. At first the justiciar was successful.

In 1225 a plot to ship Eleanor of Brittany, who as cousin to Henry III always posed a potential threat to the crown and was thus viewed as a state prisoner and then confined at Bristol Castle, away to France, was reported. The plot might have been false and only fabricated to discredit Peter, and he eventually fell out of royal favor in spring 1234.[10]

Peter participated in the Sixth Crusade alongside William Briwere, who was Bishop of Exeter.[11] An army of other crusaders accompanied them to the East, although whether they were English or mercenaries recruited on the Continent is unclear. The contingent left from Brindisi in August 1227.[12] Both bishops were influential advisors to Frederick II the Holy Roman Emperor[11] even though Pope Gregory IX had ordered that no one collaborate with Frederick, who was at the time excommunicate. Both bishops ignored the papal orders and worked closely with Frederick's agents and Frederick himself. The financial resources both bishops brought were especially appreciated by the crusaders.[11] Both bishops witnessed the treaty on 18 February 1229 with the Sultan of Cairo that restored Jerusalem to the Christians,[11] the Treaty of Jaffa.[12] After the crusade, he spent time in Italy.[6]

The Lanercost Chronicle relates that Peter, out hunting one day, encountered King Arthur, dined with him, and asked for a token of their meeting. Arthur told him to close his hand, then open it, whereupon a butterfly flew out. For the rest of his life Roches was able to repeat this miracle, so that he became known as the Butterfly Bishop.[13][14]